A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a mask, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction.
It is conventional in DUV lithographic apparatus to attach a pellicle to the mask. The pellicle is a transmissive film which is spaced a few mm (e.g. 5 mm.) away from the pattern of the mask. A contamination particle which is received on the pellicle is in the far field with respect to the pattern of the mask, and consequently does not have a significant impact upon the quality of image which is projected by the lithographic apparatus on to a substrate. If the pellicle were not present then the contamination particle would lie on the pattern of the mask and would obscure a portion of the pattern thereby preventing the pattern from being projected correctly on to the substrate. The pellicle thus plays an important role in preventing contamination particle from adversely affecting the projection of a pattern on to a substrate by the lithographic apparatus.
Although the pellicle provides a useful and valuable function, the pellicle causes an undesirable side effect in that it will itself have an effect upon the image which is projected by the lithographic apparatus on to the substrate. This is because the pellicle has a finite thickness and a refractive index which is greater than air, and thus will cause some deflection of any radiation which is not perpendicularly incident upon the pellicle.
It is desirable to provide, for example, a method which obviates, or mitigates, one or more problems of the prior art, whether identified herein or elsewhere.